An optical waveguide element typically comprises at least a substrate, a cladding layer formed on the substrate, and a waveguide—also called waveguide core—formed in the cladding layer for transmitting an optical signal. Such waveguide elements are also called integrated optical devices or opto-electronic integrated circuits.
Cladding layer, waveguide and substrate typically show different thermal expansion coefficients since different materials are used. While fabricating waveguide elements, the elements are exposed to high temperatures. These high temperatures cause stress in the waveguide element due to the mentioned different thermal expansion coefficients. This stress in turn induces birefringence which basically means that TE and TM waves of transmitted optical signals show different behaviour. As a consequence, birefringence is one of the factors limiting the performance of a waveguide element.
Birefringence B can generally be defined as B=ηTM−ηTE, wherein ηTE is the effective refractive index of a TE polarized wave and ηTM is the effective refractive index of a TM polarized wave. A TE wave is a linearly polarized wave having an electric field direction in parallel with the major surface of the substrate. A TM wave is a linearly polarized wave having an electric field direction perpendicular to the major surface of the substrate. In other words, the perpendicular to the major surface of the substrate. In other words, the refractive indexes differ from one and another by B depending on the polarization direction of the incident light.
U.S. Pat. No. 4,781,424 addresses the problem of birefringence that is induced when manufacturing a waveguide element. U.S. Pat. No. 4,781,424 proposes to provide a stress adjusting means in the cladding layer for adjusting a stress applied to the waveguide. Such means might be an elongated member which is embedded in the cladding layer and may be composed of a material having a thermal expansion coefficient that is different from the one of the cladding layer. Alternatively, the stress adjusting means might be a groove provided in the cladding layer for relieving stress.
U.S. Pat. No. 4,900,112 discloses a single-mode optical waveguide element comprising a substrate, a cladding layer disposed on the substrate and a waveguide embedded in the cladding layer. A stress applying film in form of an amorphous silicon film is disposed on a desired portion of the cladding layer for adjusting stress induced birefringence by changing the stress exerted on the waveguide by a trimming technique. The stress applying film is trimmed by partially irradiating a light beam on it. The stress is changed irreversibly.
U.S. Pat. No. 5,117,470 discloses a method for adjusting a refractive index difference between a cladding layer and a waveguide core of an optical waveguide element by means of producing a reversible thermal hysteresis phenomenon. The thermal hysteresis is evoked by raising temperature in a predefined region of the waveguide element to a predefined level, maintaining the predefined temperature for a predetermined period of time, and cooling the region at a predetermined cooling rate. This method is carried out in order to change a coupling ratio of an optical coupler.
An optical waveguide element made in thin-film technology often comprises an optical phase shifting means. A heater can serve as such phase shifting means acting on an optical signal that is guided by means of a waveguide in accordance with the thermo-optical effect, such that refraction η=η(T), with T as temperature.
In particular, an optical waveguide element can comprise two waveguides. A phase shifting means is applied to one of the waveguide cores for adjusting a phase of an optical signal in this waveguide relative to a phase of an optical signal guided in the other waveguide. When such optical signals are interfered—e.g. by means of an optical coupler—, an intensity modulated signal can be attained. Such an optical waveguide element can be used in variable optical attenuators, optical ring resonators, dispersion compensating devices, Mach-Zehnder interferometers, add-drop multiplexers, optical wavelength converters or amplitude-shift keying (ASK) as well as phase-shift keying (PSK) modulators operating particularly in low wavelength regions.
FIG. 1 shows a known waveguide element having a first and a second waveguide 3, 4 aligned in parallel, and having a first and a second optical coupler 301, 302, through which optical signals can be exchanged between the waveguides 3 and 4, and with a thin-film heater 100 covering a part of the first waveguide 3 and lying between the couplers 301, 302. An optical signal entering the first waveguide 3 at port A—indicated with an arrow—will partially be coupled from the first optical coupler 302 to the second waveguide 4. Between the directional optical couplers 301, 302, the phase of the remainder of the optical signal transferred in the first waveguide 3 will be shifted according to the thermal energy applied to the first waveguide 3 by means of the thin-film heater 100. The optical signal in the first waveguide 3 then interferes in the second coupler 302 with the optical signal of the second waveguide 4. Depending on the phase relationship between the optical signals in the waveguide sections before the coupler 302 the signal intensity in the second waveguide 4 at port D will be increased or reduced accordingly.
In order to obtain a desired shift of the phase of the optical signal in the first waveguide 3 relative to the phase of the optical signal in the second waveguide 4, thermal energy provided by the thin-film heater 100 is applied to the first waveguide 3. In the region of the thin-film heater 100, the waveguides 3, 4 are spaced apart at a distance which is sufficient to avoid a transfer of thermal energy from the thin-film heater 100 to the second waveguide 4. Thermal energy provided by the thin-film heater 100 is absorbed by a substrate 5 acting as a heat sink in such a way that the thin-film heater 100 forces a temperature gradient with respect to the substrate 5. The waveguide element according to FIG. 1 can be derived from U.S. Pat. No. 4,781,424.
Since the signal intensity in the waveguides 3 and 4 at ports C and D can vary depending on the phase relationship between the optical signals before being coupled in coupler 302, an optical waveguide element according to FIG. 1 can be used as variable optical attenuator (VOA), where constructive and/or destructive interference in the second coupler determines intensities of optical signals at the outputs of the second coupler.
Hence, thermo-optic control of a phase shift is an elegant way to realize adaptive devices in silica-on-silicon waveguide technology. A millisecond time response of the thermo-optic effect enables routing and compensation applications.
However, applicant detected some undesired polarization effects caused by such phase shifting heaters: Heating a section of a waveguide in order to obtain a phase shift causes a different phase shift efficiency for TE and TM polarized light. Local heating causes asymmetrical stress in the waveguide core. In addition, simply the presence of a heater—typically a thin film heater disposed on the top of the cladding layer—causes at least some birefringence due to its asymmetric arrangement with regard to the waveguide core.
Thus, a phase shift serving heater does not affect TE and TM waves of an optical signal in the same way due to an elasto-optical effect. As a result, the optical waveguide element cannot fulfill its task predictably, unless the polarization direction of an optical signal is previously adjusted to either a direction parallel or a direction vertical to the surface of the substrate, i.e. the input polarization with regard to the heater is not either being a linear TB or a linear TM wave.
With regard to an optical variable attenuator, a polarization-dependent loss (PDL) can be detected at the device's output that amounts to at least 0.25 dB for an attenuation of −15 dB.
It would therefore be desirable to create an optical waveguide element showing controllable phase shifting properties with regard to TE and TM waves. It would further be desirable to have a waveguide element operating with high efficiency.